None of the current answers seem to talk about how to change the colour used in highlighting.

In my config, I have tabs and trailing whitespace highlighted using whitespace-mode:

(require 'whitespace)
(whitespace-mode 1)  ;; or (global-whitespace-mode 1)
(setq whitespace-style '(face tabs trailing))
(modify-face whitespace-tab nil "#ff0000")
(modify-face whitespace-trailing nil "#ff0000")

Code for highlighting tabs obtained from a comment under this StackOverflow answer.